Answers by Carol E.

Let first number of {} = 26 and add 13 to every number after 26 stopping at 52, and eliminate any number not ending in an even integer and ending in 0 to get the sum set. Stop at 998 to get 52 as the sum of each 3-number sets. 52/13= 4. There is just 26,39, and 52 as possibilities. 998+998=52...